[QEMU PATCH v4 10/10] target/i386: kvm: Add nested migration blocker only when kernel lacks required capabilities

[Liran Alon <liran.alon@oracle.com>]

Previous commits have added support for migration of nested virtualization
workloads. This was done by utilising two new KVM capabilities:
KVM_CAP_NESTED_STATE and KVM_CAP_EXCEPTION_PAYLOAD. Both which are
required in order to correctly migrate such workloads.

Therefore, change code to add a migration blocker for vCPUs exposed with
Intel VMX or AMD SVM in case one of these kernel capabilities is
missing.

Signed-off-by: Liran Alon <liran.alon@oracle.com>
---
 target/i386/kvm.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index 99480a52ad33..a3d0fbed3b35 100644
--- a/target/i386/kvm.c
+++ b/target/i386/kvm.c
@@ -1313,9 +1313,14 @@ int kvm_arch_init_vcpu(CPUState *cs)
                                   !!(c->ecx & CPUID_EXT_SMX);
     }
 
-    if (cpu_has_nested_virt(env) && !nested_virt_mig_blocker) {
+    if (cpu_has_nested_virt(env) && !nested_virt_mig_blocker &&
+        ((kvm_max_nested_state_length() <= 0) || !has_exception_payload)) {
         error_setg(&nested_virt_mig_blocker,
-                   "Nested virtualization does not support live migration yet");
+                   "Kernel do not provide required capabilities for "
+                   "nested virtualization migration. "
+                   "(CAP_NESTED_STATE=%d, CAP_EXCEPTION_PAYLOAD=%d)",
+                   kvm_max_nested_state_length() > 0,
+                   has_exception_payload);
         r = migrate_add_blocker(nested_virt_mig_blocker, &local_err);
         if (local_err) {
             error_report_err(local_err);
-- 
2.20.1